1. Field of the Invention
The present invention relates generally to a method and an apparatus for reducing the number of bits required to convert a digital signal to an analog signal in a Frequency Division Multiple Access (FDMA) system using a subcarrier based allocation.
2. Description of the Related Art
In accordance with the recent increase in demands for not only voice communications, but also massive data services such as various multimedia Internet services in a wireless communication market, an Orthogonal Frequency Division Multiple Access (OFDMA) scheme and a Single Carrier Frequency Division Multiple Access (SC-FDMA) scheme are attracting more attention as possible wireless transmission techniques that may be used to meet those demands.
With regard to OFDMA, the OFDMA scheme typically converts serial data streams to N-ary parallel data streams and transmits the converted parallel data streams by allocating them to separate subcarriers as illustrated in FIG. 1A. The OFDMA scheme achieves efficient resource distribution by differing the number of the assigned subcarriers depending on a data rate requested by a user and avoids Inter-Symbol Interference (ISI) caused by a time delay spread by means of a Cyclic Prefix (CP). Hence, the OFDMA is quite efficient in a wireless communication system having a relatively wider cell. Disadvantageously, since the signals corresponding to the multiple subcarriers are mixed at the transmitter, the OFDMA scheme is also subject to a considerable Peak to Average Power Ratio (PAPR) of the signal.
Similar to the OFDMA scheme, the SC-FDMA scheme transmits data per subcarrier. As illustrated in FIG. 1B, the SC-FDMA scheme conducts a Discrete Fourier Transform (DFT) 110 before an Inverse Fast Fourier Transform (IFFT), to thus lower the PAPR, which is the shortcoming of the above-described OFDMA scheme. The OFDMA maps the data in the frequency domain prior to the IFFT, whereas the SC-FDMA maps the data in the time domain prior to the DFT 110 preceding the IFFT. Since the mapped data are transmitted with the single carrier characteristic sustained, the PAPR is reduced in spite of the multicarrier. The SC-FDMA scheme is determined as the standard of the 3rd Generation Partnership Protect (3GPP) Long Term Evolution (LTE) uplink which is a future communication system. Both the OFDMA and the SC-FDMA allow the data allocation per subcarrier within a symbol. This SC-FDMA scheme method can acquire efficiency by broadening the width of the data allocation, but disadvantageously at the expense of increasing the signal dynamic range; that is, the variation of the signal level.
For example, referring now to FIGS. 2A and 2B, in the OFDMA scheme, the transmit signal power level in the data allocation to 1024 subcarriers of FIG. 2B is greater than the transmit signal power level in the data allocation to 256 subcarrier of FIG. 2A. The greater number of the assigned subcarriers, the greater energy of the signal transmitted in the same time.
In a system requiring the high-speed data transmission such as OFDMA system and SC-FDMA system, the increase of the transmission bandwidth increases the size of a Fast Fourier Transform (FFT) and also raises the ratio of maximum allocated subcarriers to minimum allocated subcarriers; that is, the allocation ratio.
In a conventional system, the allocation ratio affects the number of bits needed to convert the digital signal to the analog signal (hereinafter, referred to as a Digital-to-Analog Conversion (DAC) bits). Accordingly, the higher allocation ratio requires a greater number of DAC bits.
The number of the DAC bits in the conventional system varies according to the allocation ratio as expressed in Equation (1).DAC bits=ceil(ENOB)+1ENOB=(Psig−1.76)/6.02Psig(dB)=SNRreq(dB)+PAPR(dB)+Ralloc(dB)+Margin(dB)  (1)
In Equation (1), “ceil” denotes a function which rounds up to the nearest integer, ENOB denotes the effective number of bits, and +1 denotes a value corresponding to a sign bit. Psig denotes a dB scale value, SNRreq(dB) denotes a required Signal to Noise Ratio (SNR), and PAPR(dB) denotes a Peak to Average Power Ratio (PAPR) of the signal. Ralloc(dB) denotes a ratio of minimum allocated subcarriers to maximum allocated subcarriers of the signal and Margin(dB) denotes a margin in consideration of noise and distortion caused by nonlinear characteristic of the DAC.
Now referring to FIG. 3, to calculate the number of the DAC bits required at the transmitter based on Equation (1), Psig(dB) 311 is obtained by summing up SNRreq(dB) 305, PAPR(dB) 303, Ralloc(dB) 301, and Margin(dB) 307 and converted to ENOB 313. The converted ENOB 313, which will undergo DAC conversion 315, is rounded up to its nearest integer to produce the effective bits 317 and added with the sign bit 319. As a result, the number of the DAC bits 315 can be converted. For example, given SNRreq 30 dB, PAPR 10 dB, Ralloc 20 dB, and Margin 10 dB, Psig is 70 dB, ENOB is 11.34, and the effective bit is 12. Accordingly, the DAC bit is 13 including the sign bit 1. Herein, when the required SNR, the PAPR, and the allocation ratio Ralloc indicative of the signal characteristics need to transmit a plurality of different signals, the number of the required DAC bits is determined based on the signal having the greatest Psig value among the different signals.
As discussed above, the conventional method determines the number of the DAC bits in consideration of the subcarrier allocation ratio and the number of the DAC bits of the transmitter is set to the number of bits required in the highest subcarrier allocation ratio. However, since the subcarrier allocation ratio is variable, every DAC bit is not used in case of lesser allocations. In great allocations, the number of bits greater than needed is not utilized. As a result, the efficiency degrades for the number of the DAC bits designed. The degraded efficiency ultimately increases the unit cost of the DAC, the power consumption, and the hardware size due to the increased filter bits. Thus, there is a need in the art to provide a method for reducing the DAC bits without degrading the performance.